308 Roque et al.

The distribution of organisms varies in space and time, and is strongly influenced by environmental heterogeneity. Changes in temperature, humidity, and resource availability, among other factors, can alter the size, density, and distribution of populations. A portion of these variations has evolutionary meaning, because it can alter the structure and genetic composition of the populations. However, although the relationships between organisms and environments are a central issue in ecology, they are still poorly understood (Begon et al. 2006). Drosophilids are considered excellent models for biological research (Powell 1997), but little is known about their ecology, especially in tropical environments (Val et al. 1981; Tidon 2006). These flies breed in a wide variety of organic substrates, including fallen fruits, flowers, slime fluxes, and fleshy fungi (Carson 1971). During their decomposition, these substrates attract flies due the presence of microorganisms, principally yeasts, which constitute the alimentary base for these insects Breeding sites of drosophilids (Diptera) in the Brazilian Savanna. I. Fallen fruits of Emmotum nitens (Icacinaceae), Hancornia speciosa (Apocynaceae) and Anacardium humile (Anacardiaceae